The present invention relates to a process for fabricating a transistor, more specifically, a heterojunction bipolar transistor and a tunneling hot electron transistor (HET).
Heterojunction bipolar transistors, in which the emitter bandgap is wider than the base bandgap, have the advantage of higher emitter efficiency since holes (minority carriers for the emitter) flowing from the base to emitter are blocked by the higher barrier in the valence band (cf. W. Shockley, U.S. Pat. No. 2,569,347). The wide bandgap emitter allows the base to be heavily doped without sacrificing emitter efficiency. However, the cutoff frequency is relatively low, normally, a few gigahertz.
A typical heterojunction bipolar transistor is fabricated, by sequentially forming on a substrate, a collector layer, a base layer, and an emitter layer. An emitter electrode is formed on the emitter layer. A base electrode is formed on the base layer, on the same side of the substrate as the emitter electrode near the emitter electrode. However, the base layer is extremely thin, for example, from 50 nm to 100 nm, formed by a process of molecular beam epitaxy (MBE) or metal organic cheemical vapor deposition (MOCVD). The distance from the base electrode or base contact region to the emitter electrode or emitter contact region is, for example, from 1 .mu.m to 2 .mu.m. As a result, the base resistance becomes high, which limits the cutoff frequency, usually to a few gigahertz.
Since the MOMOM (metal-oxide-metal-oxide-metal) hot electron device was proposed by C. A. Mead (Proc. IRE 48, 359 (1960)), considerable efforts have been made, with limited success; due to the difficulty in obtaining very-thin metal and oxide films with a high quality. A heterojunction hot electron device, proposed by M. Heiblum (Solid-St. Electron. 24, 343 (1981)), is now possible since recent progress in MBE or MOCVD technology has enabled the forming of high-quality, very-thin semiconductor films and well-controlled hetero-interfaces.
A typical tunneling hot electron transistor (HET) using heterojunctions, for example, GaAs/AlGaAs, is fabricated by sequentially forming on a substrate a collector layer, a collector-side potential barrier layer, a base layer, an emitter-side potential barrier layer, and an emitter layer. An emitter electrode is formed on the emitter layer. A base electrode is formed on the base layer, on the same side of the substrate as the emitter electrode, near the emitter electrode. In order to increase the current gain, the thickness of the base layer should be reduced. However, if the thickness of the base layer is reduced, part of the base layer between the emitter contact region and the base electrode may be fully depleted due to interface states existing there, greatly increasing the base resistance and thus making the operation of the transistor impossible.